Typical examples of such cooling units are diverters and limiters for fusion reactors of the type described, for example, in DE-PS 34 16 843. In such typical cooling units, graphite is preferably used for the parts comprising heat-resistant material, such parts being connected to the coolant conduits--which preferably are made of molybdenum--by a soldered joint. For this purpose, silver-copper solders are used as the solder material. Such solders are adequately capable of withstanding the high thermal stresses acting on the soldered joint.
A disadvantage of the above-described cooling units is that only relatively expensive materials (such as molybdenum or molybdenum alloys) can be used for the coolant conduits, which preferably have walls as thin as possible for good heat transfer. The use of more favorably priced materials with good thermal conductivity, however, cause considerable problems. For example, copper or copper alloys pose a problem in that the silver-copper base solders used for soldering form a low-melting eutectic with the copper or the copper alloy, which, in the soldering process, causes the coolant conduit to start to melt, or may even lead to local dissolution of the coolant conduit in the process. With nickel-iron or nickel-iron-cobalt alloys, on the other hand, cracking of the material may occur, especially under high alternating thermal stresses, which reduces the useful life of the cooling unit.
EP-A1-59 960 relates to a cooling system for blast furnaces, wherein cast steel cooling elements are joined with steel coolant conduits by recasting the finished coolant conduits in a suitable mold with liquid steel for the cooling elements. In order to prevent the coolant conduits from melting, the coolant conduits are provided on the surface with a coating comprising ceramics or zirconium.
U.S. Pat. No. 4,532,101 describes a limiter element with three sections for a fusion reactor. The individual sections consist of a rigid supporting part, on which several graphite plates are mechanically fastened, such plates being coated with titanium carbide. The supporting part is made of stainless steel and is also soldered to the coolant conduit which is made from a steel alloy.